This invention relates to a membrane press, and a method of elevating a workpiece above the supporting surface of a work base, such as a press table. Membrane presses have been in existence since the early 1900's, and are used for applying a uniform, laminating exterior veneer to the top surface, corners, and peripheral borders of a relatively flat, three-dimensional workpiece. The resulting laminated workpieces are typically used in furniture fronts, doors, cabinetry, desks., and other similar items where pleasing outward appearance and durability is desirable.
The production of dome-type membrane presses for laminating a workpiece with a veneer began in the 1930's. These type presses were effective for laminating particularly bent or curved furniture parts. However, the later development of a durable, wood-based MFD board, and the trend toward furniture designs with flat surfaces and sharp edges rapidly diminished the industry demand for dome-type presses. In response to the new furniture trends, a new membrane press was developed incorporating the design of a flat press. Flat presses have larger dimensions and allow increased production. Today, dome-type membrane presses are seldom used for laminating, and are only necessary for very curved workpieces.
The pressing cycle of the present-day membrane press, using combined contact and convection heating, takes place in essentially four stages. The membrane press is first opened, and the laminating foil clamped along respective edges to a tensioning frame located directly above the workpiece to be laminated. The tensioning frame includes a top heating platen and membrane located adjacent to and above the laminating foil. The membrane is sucked upwardly by a vacuum source to the top heating platen which acts to heat the membrane.
In a second stage, the membrane press is closed over the workpiece, and the membrane ventilated through exhaust passages. Hot air is circulated between the top heating platen and membrane to further heat the membrane. The foil is sucked upwardly into contact with the heated membrane by a second vacuum source to plastify the foil.
Thirdly, a third vacuum source is established under the plastified foil to prevent air pockets from forming between the foil and exterior surface of the workpiece. The hot air circulated between the top heating platen and membrane is replaced by pressurized heated air to force the membrane and laminating foil downwardly onto the surface of the workpiece.
Finally, separation air is injected between the foil and membrane to lift and separate the membrane from the foil. After pressing, any excess foil overlapping the peripheral borders and corners of the laminated workpiece is removed by cutting. An addition surface finish or coating may then be applied to the workpiece to create a finished product ready for use in a furniture front, or similar article.
In order to properly and uniformly apply the laminating foil to the peripheral borders and corners of the workpiece, as described above, the workpiece must be slightly elevated above the supporting surface of the press table. According to prior art presses, a relatively thin particle board, often referred to as a dummy board, piggyback board, or raiser panel, is placed under the workpiece. These dummy boards limit the production of the press, since the corners of the boards often crumble during pressing. The crumbled particles then become positioned under the foil, which ultimately results in workpiece rejects because the particles show through the foil. Moreover, the dummy boards must be shaped and sized to fit the dimensions of each particular workpiece.
The present invention is an alternative means for elevating a workpiece above the supporting surface of a work press, such as a membrane press. The present invention utilizes the already-existing vacuum force acting beneath the workpiece and foil to elevate the workpiece. This results in increased production of the press, and reduces the number of workpiece rejects.